This invention pertains to a method and apparatus for modifying a video signal so that a television receiver produces a normal color picture from the modified signal, whereas a videotape recording of the modified signal produces generally unacceptable pictures.
There exists a need for a method and system for modifying a video signal so that the signal produces a normal color picture on a television receiver, but videotape recording of the video signal is inhibited or prevented. In a copending application by the present inventor and assigned to the assignee of the present invention, entitled METHOD AND APPARATUS FOR PROCESSING A VIDEO SIGNAL, and filed in the United States Patent, and Trademark Office on Nov. 14, 1983, Ser. No. 551,696, now U.S. Pat. No. 4,577,216, there is described one way of modifying a video signal so as to inhibit videotape recording thereof. In that copending application, the color burst component of a video signal is phase modulated, which causes a videotape recorder to interpret the phase variation as a velocity error and alter the chrominance signal, giving rise to color noise in the videotape recording. In accordance with the present invention, it has been discovered that there are other ways to inhibit videotape recording of a video signal, by modifying the video signal.
Video cassette recorders designed for the consumer market place invariably feature some form of automatic level control (ALC). The ALC circuit insures that the video level applied to the FM modulator in the recording system remains at a fixed, predetermined value, even if the video level applied to the machine's input terminal varies widely about the nominal value. Without an ALC system, high level signals would be distorted and clipped and low level signals would be marred by the noise and interference products generated by the recording process. If the input level dropped to less than 1/3 of normal value, the replayed signal might not even be strong enough to reliably synchronize the receiver's timebases, hence giving rise to noisy unstable pictures.